Said another way: The phototransistor sensor would need to have a sensing diameter (field of view) equal to or smaller than the narrowest slit width. It is my impression that the generally available sensors have a fairly wide diameter (angle of acceptance). If my assumption is correct then the described method will not work

Mike.

Regardless of sensor size, if you measure the time between half-max output and half max output of the sound card signal, you'll still get a reasonably good reading. It would only reach some percentage of potential max output, let's say 5% for example, so your shutter speed would be the time between 2.5% max potential beginning and ending outputs from the sensor. What you're assuming is that an aperture/slit covering only part of the sensor will drive the sensor output to zero, which isn't correct.